A. Field of the Invention
This invention relates to bonding of glass-like structures to one another and to metallic structures, particularly where hermetic sealing is desired.
B. Description of Prior Art
Numerous methods exist for forming bonds between glass-like (i.e., glass, quartz, fused silica, fused alumina and other substantially non-porous) structures to one another and to metallic structures. These previously applied methods tend to fall within three categories:
The structures may be bonded by fusing. In effect this constitutes heating the junction to a high temperature and forming a weld between the structures. While this method will produce a "clean" bond, i.e., one without contamination, it is a very high temperature process which may not always be possible. This requires close matching of coefficients of thermal expansion. Because of this the bond, once created, may not withstand thermal cycling. Also, particularly in the case where the bond is intended to create a hermetic seal for a container, the high temperatures necessary to create the bond may adversely affect the material being sealed within the container.
The bond may also be formed by soldering the two structures. Here an inter-layer is introduced between the two structures and, by application of high temperature which melts the inter-layer, the two structures are joined. This method is superior in some applications to the method employing fusing, since the inter-layer can be selected to have an intermediate coefficient of thermal expansion between the two structures, which permits a greater resistance to thermal cycling damage. However, it is also a rather high temperature process and is not, therefore, practical for many applications.
Finally, the mating surfaces of the two structures (or at least the non-metallic one(s)) may be metalized and the structures may be metallically brazed or soldered together. This presents some of the advantages of the soldering method described above, in that the solder or brazing compound selected may be of intermediate coefficient of thermal expansion. However, like the foregoing two methods, this is a rather high temperature process. Also, this method introduces the problem of flux residue and the fact that low temperature solders are not noble materials. Thus, this method is very susceptible to contamination of the joint and/or of the encapsulated material.
Numerous patents describe and claim bonding methods which attempt to overcome some of these difficulties.
In U.S. Pat. No. 3,657,076, the inventor deals with the task of bonding quartz to steel, the problem being a greatly dissimilar coefficient of thermal expansion. The method disclosed involves vacuum depositing layers of metal to the quartz, each succeeding layer being closer to the thermal expansion coefficient of steel. The final layer is electro-plated to the steel. While this method may appear to solve the thermal expansion problem, the bond would appear to have little structural integrity and, because of this inherent weakness of the bond, would probably not survive a very wide range of thermal cycling, at least if the thermal cycling were combined with mechanical stress.
U.S. Pat. No. 1,090,456 teaches a method of joining a conductor to an insulator. Here, a glass part is metalized by dipping to form a metal glaze or metallic paste fired into the surface. The metallic part is then put in contact with the metalized glass, and a thin film of metal is plated over the entire mass. This would tend to seal the glass-metal interface. However, the structure would have very little mechanical strength and would probably not survive the thermal cycling.